Field of the Invention
The invention relates to elevators, elevator safety arrangements, and a method and an elevator for stopping an elevator car using elevator drive.
Description of the Related Art
Elevator brakes are an extremely important safety feature. Despite the use of a counterbalance, free falling of a traction elevator car either upwards or downwards may have detrimental effects. The counterbalance is sized to have a mass of an elevator car with 50% load. With such a choice of counterbalance, an empty elevator car or an elevator car with only a single passenger or a few passengers is more likely to accelerate uncontrollably upwards in case no brakes are applied. The movement of an elevator car may be slowed down by a worm gear, if the elevator motor uses gears. However, with the introduction of gearless elevator motors, the acceleration becomes higher. Elevator shafts may be equipped with buffers which comprise, for example, springs. The problem with buffers is that in elevators with limited upper or lower space it is not possible to install buffers that would provide safe deceleration. This is usually due to the fact that elevators may be installed in old buildings where it is not possible to reserve an entire top or bottom floor for buffers only. Further, it may be difficult to change a building afterwards so that structures sufficient to mount heavy impact buffers could be built. In many cases buffers are capable of absorbing speed up to 60% of the maximum speed.
Due to these factors elevator brakes are designed with pronounced fault-tolerance. Brakes associated with a traction sheave are usually duplicated. The design of the brakes is such that sudden loss of electrical power does not result into a failure of the brakes. When power supply to elevator brakes interrupts, the elevator brakes close mechanically. This involves that elevator brake disks or pads grip the traction wheel. In addition to traction wheel brakes, an elevator car may be equipped with grippers that grip elevator car tracks in the elevator shaft in order to brake the elevator car. The general purpose of the tracks is to keep the elevator car steady and inhibit swinging of the elevator car when being hoisted with the traction wheel. Elevators are also equipped in an overspeed governor, which consists of an overspeed governor wheel, governor ropes connected to the elevator car and the counterbalance, and a sheave. In the event of a significant overspeed centrifugal force causes the overspeed governor wheel to pull a braking wire which in turn causes wedge-shaped brakes to engage the elevator car tracks. The problem with braking the elevator car using grippers or the overspeed governor is that the deceleration may become rapid. The resulting torque may feel unpleasant. Further, gripping procedure is irretrievable such that when the gripping has taken place, a serviceman has to visit the elevator site to restore the elevator operation and release the passengers from the elevator car. Usually, elevator car grippers are applied in extreme overspeed or fault situations.
Despite the fact that traction sheave brakes are duplicated, fault situations may occur where both brakes fail simultaneously. A possible such situation may occur, if the brakes have been disabled manually during maintenance or inspection.
In prior art elevator safety circuits have only made it possible to cut power supply to an elevator. This has resulted in a situation where only mechanical safety measures are available for braking the elevator car. However, with the introduction of processor controlled elevator safety systems, it has become possible to apply more sophisticated safety measures.
Due to the aforementioned problems, it would be beneficial to be able to stop an elevator car more gracefully. Further, it would be beneficial to be able to introduce a further measure of safety for the stopping of an elevator car at the event of a failure.